The present invention relates to a process for selecting an antenna in a wireless communication system having at least two antennas. In particular, the present invention is designed to select the best antenna based on real and time delayed quality indicators.
In wireless communication systems, incoming signals often experience time dispersion and fading which is referred to as multipath effect. Multipath effect occurs when a signal and its replica arrive at a receiver out of phase to any extent. Generally, multipath effect occurs because artificial and natural obstructions cause transmitted signals to follow several propagation paths from a transmitter to a receiver. In wireless technology, the phase differences of the signals arriving at the receiver produce background noise, and in severe cases, cause the receiver to squelch. Conventional wireless technology utilizes antenna diversity to minimize this problem. A wireless network commonly includes more than one antenna positioned such that the signals received by the antennas are not correlated.
In DECT (Digital Enhanced Cordless Telephony) and PWT (Personal Wireless Telephony) systems, for example, two antennas are available in the base station and/or portable. FIG. 1 and FIG. 2 illustrate a conventional wireless system configuration that includes a base station and a portable having two antennas.
When more than one antenna is used in a wireless system, a protocol is employed to select the best antenna. Normally, antenna selection is based on a singular quality indicator adapted from the field of cryptography. The kinds of quality indicators may be divided into two categories namely: (1) those which are designed to authenticate signal transmissions and select an antenna as the signal is received and (2) those which are designed to authenticate signal transmissions and select an antenna after the signal is received. Known devices of both types have drawbacks and disadvantages.
In a TDMA-based system, for example, the antenna selection is controlled by software or logic circuitry. In this system, a cyclical redundancy check (CRC) like parameter is generally used to select an antenna after the signal is received. CRC is based on polynomial division in which each bit of a packet of data represents one coefficient of a polynomial. The polynomial is then divided by a pre-programmed polynomial to yield a quotient polynomial and in some cases a remainder polynomial. When the division yields a remainder polynomial., the system assumes that a transmission error occurred and selects another antenna. If, however, the division does not yield a remainder polynomial, the system assumes no transmission errors occurred and therefore does not select another antenna.
The antenna selection process is illustrated in FIG. 3. A CRC error rate that produces good speech quality is used as a threshold for selecting an appropriate antenna. If the present antenna provides a CRC error that is below the threshold value, no switching occurs. However, when the CRC error rate rises above the threshold value, another antenna is selected.
While CRC provides antenna selection by authenticating transmitted data, it has disadvantages. Its primary shortcoming is that antenna selections are not made in real time. The present antenna selected is based on a previous CRC comparison which does not change until the antenna receives a poor quality signal. The time delay that exists between receiving an incoming signal and selecting another antenna makes the selection process susceptible to error due to interference. A CRC selection may be accurate if a transmitter or receiver is stationary or moves at a slow rate of speed, because the communication environment is subject only to slight variations in time. However, when a transmitter or receiver moves at a high rate of speed, this time delayed process may be ineffective because it may not react to a changing environment and thus, it may be susceptible to interference.
Another technique for antenna diversity switching authenticates signal transmissions and selects an antenna as the signals are received. Preamble diversity switching is an example of a system that provides real-time measurements and real-time antenna selection. Preamble diversity switching sequentially measures the receive signal strength of a diversity of antennas at the beginning of each extended preamble. The receive signal levels of each antenna, which are the Receive Signal Strength Indicators (RSSI), are stored and compared. The antenna with the higher RSSI value is selected. When the RSSI value associated with another antenna is higher, that antenna is then selected. An example of a preamble diversity switching process is shown in FIG. 4.
The preamble diversity switching process provides the benefit of selecting an antenna as signals are received. The system is less affected by rapid environmental change. However, problems arise when differences between RSSI values are insignificant. When insignificant differences exist, the system may experience some uncertainty when selecting an antenna. This is simply because minor differences in RSSI values indicate that the signal qualities received by the antennas are similar and therefore, an of antenna selection will not necessarily improve receiving quality. Therefore, a preamble diversity switching process alone may not be the best parameter for selecting an antenna. An additional indicator may be desirable to select an optimal antenna.
It is therefore an object of the invention to provide an improved antenna selection process for a TDMA-based telephone communication system. In this disclosure, a: TDMA/PWT based system is illustrated.
Another object of the invention is to implement an appropriate decision strategy that considers the limitations of the signal authenticating processes and the error rates of signal transmissions.
Still another object of the invention is to provide an improved antenna selection process for TDMA-based systems that may employ more than two antennas.
Pursuant to the invention, a wireless antenna selection and switching system is provided which is capable of selecting an antenna from a diversity of antennas. The antenna diversity switching system is capable of measuring signal quality. When signals are received from a plurality of antennas, the wireless antenna selection and switching system automatically selects a real-time or a time-delayed signal authentication process. The chosen process then selects the antenna that will provide the clearest signal quality.